1. Field of the Invention
The present invention relates to a catalytic process for preparing oximes. In this process, a carbonyl compound, preferably a cycloalkanone having from 7 to 20 carbon atoms, is reacted in the liquid phase with ammonia and hydrogen peroxide (ammoximation), over a heterogeneous catalyst system comprising two or more components of which at least one of the components comprises at least one porous, titanium-containing solid, and at least one second component comprises an acidic solid.
2. Discussion of the Background
European patent applications EP-A-0 208 311, EP-A-O 267 362 and EP-A-0 299 430 and U.S. Pat. No. 4,794,198, each of which is herein incorporated by reference, describe the preparation and activation of a catalyst based on titanium, silicon and oxygen, and its use for the synthesis of oximes from aldehydes or ketones, for example cyclohexanone, by reaction with hydrogen peroxide and ammonia. The catalysts usually have a silicon:titanium ratio of greater than 30. A typical representative catalyst is the titanium silicalite TS1.
While the synthesis of relatively small aliphatic and cycloaliphatic oximes from ketones having up to 6 carbon atoms, for example, cyclopentanone and cyclohexanone, gives good results for numerous titanium silicalite catalysts, prepared and activated as described in the above mentioned documents, the results are significantly poorer when larger or more sterically hindered carbonyl compounds, such as acetophenone and cyclododecanone, are used. In particular, the reaction rate, the percent conversion of carbonyl compound used, and the hydrogen peroxide selectivity (H2O2 used for the ammoximation/total amount of H2O2 requiredxc2x7100%) are unsatisfactory in these experiments.
In the examples of EP-A-O 267 362, conversions of over 90% at a peroxide loss of below 10% are achieved for cyclohexanone (Examples 22 and 24). Comparable reaction conditions using acetophenone give conversions of only 50.8% at a peroxide loss of 48.9%. The reaction of cyclododecanone is also claimed in the cited application, but no specific example is provided with regard to the conversion and peroxide loss obtained when reacting cyclododecanone.
The significantly poorer results obtained for large or sterically hindered carbonyl compounds can be attributed, inter alia, to the inability of large carbonyl compounds such as cyclododecanone to penetrate, or their ability to penetrate only slowly, through the pores of the titanium silicalite catalyst. This can lead to spatial separation of the substeps of hydroxylamine formation (1) and oximation of the ketone (2) (in the reaction equations shown below for cyclododecanone (CDON)).
The decomposition of hydroxylamine by reaction with hydrogen peroxide, formally represented by the stoichiometric equation (3), can occur to a considerable extent as a competing reaction, which reduces the productivity of the reaction and the hydrogen peroxide selectivity.
NH3+H2O2xe2x86x92H2O+NH2OHxe2x80x83xe2x80x83(1) 
NH2OH+CDONxe2x86x92CDON oxime+H2Oxe2x80x83xe2x80x83(2) 
2 NH2OH+H2O2xe2x86x924 H2+N2xe2x80x83xe2x80x83(3) 
German patent application DE 195 21 011 A1 (corresponding to U.S. Pat. No. 5,498,793), describes an amorphous silicon dioxide cocatalyst for the ammoximation of acetophenone and cyclododecanone, in which the addition of amorphous silicon dioxide provides for an increase in the conversion of cyclododecanone after a reaction time 8 hours to 85.5% or 85.2% (DE 195 21 011, Examples 5 and 6) compared to 76.6% without the cocatalyst. The peroxide yield at the same time increased from 65.8% to 71.4% or 72.3%. This process leads to a slight improvement in conversion and peroxide yield, but it also has a number of disadvantages which would make it uneconomical for industrial use:
The amount of catalyst and cocatalyst based on the ketone used is very high in the examples, namely up to 25% by weight in each case, for reactions using cyclododecanone as a starting material.
Despite the high catalyst concentration, the conversion rate is low and the reaction is slow.
Even after a total reaction time of 8 hours, the oxime yield is still far from complete conversion (i.e., complete conversion means an oxime yield of about 99%, preferably above 99.5%).
The mean conversion rate over a reaction time of 8 hours is 7.10 to 7.3 mg of oxime/(g of catxc2x7min) compared to 6.38 mg of oxime/(g of catxc2x7min) without the amorphous silicon dioxide cocatalyst.
For relatively large rings such as, for example, cyclododecanone, high conversion rates, which lead to complete conversion, are very important for industrial applications, because as the molecular weight increases, it is technically difficult to separate the unreacted ketone from the corresponding oxime.
It is therefore an object of the present invention to provide a process in which the ammoximation proceeds with virtually complete conversion combined with a high conversion rate and good peroxide yield. The percent conversion of the carbonyl compound to an oxime should, where possible, be so high that a subsequent reaction of the carbonyl compound with an aqueous hydroxylamine solution may be dispensed with. It has surprisingly found that this object can be achieved by reacting a carbonyl compound, hydrogen peroxide and ammonia in the presence of an acidic cocatalyst together with the titanium-containing catalyst. In particular, it has been found that the conversion rate can be significantly improved thereby.
Thus, an embodiment of the present invention provides for reacting a carbonyl compound, hydrogen peroxide and ammonia in the presence of a catalyst system comprising a catalyst and a cocatalyst, wherein the catalyst comprises at least one crystalline microporous or mesoporous solid comprising titanium, silicon and oxygen, and the cocatalyst comprises an acidic solid comprising an organic or inorganic support material, and the support material itself has Lewis-acid or Brxc3x6nsted-acid properties, or Lewis-acid or Brxc3x6nsted-acid functional groups are physically or chemically applied to the support material.
The catalyst is preferably a compound comprising titanium, silicon and oxygen, and having a porous structure, for example titanium silicalites. The porous structure may be either microporous and/or mesoporous structures. By microporous structure, we mean a structure having pores sizes which are less than 2 nm. By mesoporous structure, we mean a structure having pore sizes in the range of approximately 2 to 50 nm. Non-limiting examples of microporous titanium silicalites are the types TS1 and Ti-beta. Non-limiting examples of mesoporous structures are the titanium silicalites of the type Ti-MCM41 and Ti-HMS. The preparation of TS1 type silicalites is described, for example, in U.S. Pat. No. 4,410,501 and Bruno Notari, xe2x80x9cMicroporous Crystalline Titanium Silicatesxe2x80x9d, Advances in Catalysis, vol. 41 (1996), pp. 253-334; the preparation of Ti-beta is described, for example, in Spanish Patent 2037596; the preparation of Ti-MCM41 is described, for example, in EP 0655278; and the preparation of Ti-HMS is described, for example, by Tanev et al, Nature, 368 (1994), pp. 321-323, each of which is incorporated herein by reference.
Suitable cocatalysts are solids which themselves have Lewis and/or Brxc3x6nsted-acid properties on their surface or in the pores thereof. Non-limiting examples of such inorganic cocatalysts which have Lewis and/or Brxc3x6nsted acid properties are acidic aluminum oxides and acidic, activated aluminosilicates such as bentonite, montmorillonite and kaolinite.
Alternatively, the cocatalysts may have Lewis acid and/or Brxc3x6nsted acid functional groups, either chemically or physically applied thereto. Cocatalysts having chemically applied acid groups include sulfonated or phosphonated resins. Alternatively, the cocatalyst may be an inert solid support having a physically applied acidic coating, such as a coating of a sulfonated resin or an acidic inorganic material, such as acidic aluminum oxides and acidic, activated aluminosilicates such as bentonite, montmorillonite and kaolinite on an inert solid support. The inert solid support may be any organic or inorganic material which is not affected by the ammoximation reaction conditions or reagents used. Non-limiting examples of cocatalysts based on organic support materials which have Lewis and/or Brxc3x6nsted acid functional groups are acid and strong acid ion exchange resins such as sulfonated polystyrene ion-exchange resins, for example Amberlyst 15, or sulfonated perfluorocarbon ion-exchange resins such as Nafion NR50.
The catalyst and cocatalyst may have any physical form. For example, both may independently be a solid such as a powder or a shaped body. A shaped body is prepared by pressing a powder of the catalyst and/or cocatalyst together into the desired shape, for example using a press or by an extrusion process. The weight ratio of catalyst to cocatalyst is usually in the range from 0.1:1 to 10:1, preferably 0.5:1 to 4:1.
If the catalyst and/or cocatalyst are used as shaped bodies, it is possible for additional additives, such as binders, to be present in the shaped body. Non-limiting examples of such additives are neutral and/or weakly acidic silicates, aluminosilicates and clay minerals. In a particularly preferred variant of the invention, an acidic solid simultaneously performs the functions of a cocatalyst and of a binder in a titanium silicalite shaped body. Of course, both the catalyst and the cocatalyst can each consist of mixtures of two or more components.
The process of the present invention provides for the ammoximation of large carbonyl compounds and preferably of large cyclic ketones, in particular of rings having from 7 to 20 carbon atoms, most preferably of cyclooctanone and cyclododecanone, using hydrogen peroxide and ammonia.
The ammoximation of cycloalkanones according to the process of the present invention proceeds highly selectively. At complete conversions, the selectivity of forming the oxime is, according to analysis by gas chromatography (GC), over 99% for both cyclooctanone and cyclododecanone. If technical-grade cyclododecanone is used, the only by-products detected in the gas chromatogram are traces of cyclododecane and cyclododecanol which were originally present as contaminants in the cyclododecanone. In a few cases, laurolactam was found as a further by-product, in concentrations of  less than 0.1%.
If the reaction is carried out in a solvent, the solvent may be any compound which is stable toward hydrogen peroxide and ammonia, and sufficiently solvates both the carbonyl compound and the oxime product formed. The solvent may be miscible with water, but does not have to be. The preferred solvents are aliphatic alcohols which are miscible or partially miscible with water, selected from among C1-C6-aliphatic or cycloaliphatic alcohols, for example methanol, ethanol, n-propanol, isobutanol, tert-butanol or tert-amyl alcohol. Particularly useful solvents for the ammoximation of cyclododecanone are methanol, ethanol and tert-butanol.
Hydrogen peroxide is preferably used as an aqueous solution in commercially available concentrations (30 to 70 wt. %, preferably at least 35 wt. %). Ammonia is introduced into the reactor either as a concentrated, aqueous solution (preferably xe2x89xa720%) or preferably as a gas. When the ammonia is introduced in gaseous form and highly concentrated peroxide solutions are used, advantages result from the reduced amount of water which has to be separated from the solvent during work-up of the reaction mixture.
The reaction temperature in the ammoximation according to the present invention is from 20xc2x0 C. to 150xc2x0 C., preferably from 50xc2x0 C. to 120xc2x0 C. and particularly preferably from 60xc2x0 C. to 100xc2x0 C. The reactor is operated either at autogenous pressure, namely the pressure established as a result of the sum of the partial pressures at the respective reaction temperatures, or at increased pressure, preferably from 1 to 10 bar. The increased pressure can be provided by pressurizing the reactor with ammonia gas or an inert gas such as nitrogen. If the reactor is closed, the pressure increases slowly during the reaction due to formation of gaseous decomposition products (in particular nitrogen) during secondary reactions. It is advantageous to operate the reactor isobarically by allowing gaseous decomposition products to escape in a controlled manner via a gentle waste gas stream and replacing the ammonia which also escapes by means of a regulating valve.
Gaseous ammonia present in the waste gas stream can be collected by condensation and returned to the process.
In the ammoximation reaction, the carbonyl compound and hydrogen peroxide can each be introduced into the reactor either continuously or discontinuously. Since decomposition reactions as described by equation (3) always occur, complete conversion of the carbonyl compound requires the use of a stoichiometric excess of peroxide. The amount of excess peroxide used can be minimized by means of appropriate reaction conditions and by use of catalyst systems according to the present invention. Experimentally, it has been found to be advantageous either to charge the carbonyl compound into the reactor at the beginning of the reaction (i.e., discontinuous addition) or to meter it into the reactor in molar amounts corresponding to the amount of hydrogen peroxide added (i.e., continuous addition), and to add the necessary excess of peroxide as required by the consumption of hydrogen peroxide after addition of the carbonyl compound is complete.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.